User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicinal product is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfil a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling. To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need to be kept to a minimum.
Document US 2005/0004529 A1 for instance discloses a drive mechanism for use in a drug delivery device comprising an epicyclic gear box. This drive mechanism further has a piston rod of non-circular cross-section, a dose dial sleeve threadedly engaged with a housing and a drive sleeve located between the housing and the piston rod. Further, the dose dial sleeve is releasably connected to the drive sleeve by the epicyclic gear box. For setting of a desired dose to be delivered, a user rotates a second part of the dose dial sleeve. During such a dose setting, the dose dial sleeve, the drive sleeve and the sun gear of the epicyclic gear box are coupled together and the planetary gear cannot rotate. Also, when the dose dial sleeve is rotated, the piston rod remains in contact with a second, proximal face of cartridge's piston.
For dispensing of a selected dose, a user depresses the dose button. Displacing the dose button towards a first end of the device couples the dose button to the sun gear to lock the sun gear in rotation to the dose button. Further, a shoulder on the sun gear locking a clutch means between the drive sleeve and the dose dial sleeve is disengaged, thereby allowing the drive sleeve to rotate with respect to the dose dial sleeve. During dose dispensing, the planetary gear then creates a gear ratio between the sun gear and the dose dial sleeve.
Such known drive mechanism requires a clutch mechanism for a releasable connection of dose dial sleeve and drive sleeve by an epicyclic gear box. However, setting of particularly small doses as well as dispensing an amount of the medicinal product smaller than a standard dose unit is almost not possible.
Particularly in the field of paediatrics, there is a need of administering particularly small doses of a medicinal product.
It is therefore an object of the present invention, to provide a drive mechanism and a drug delivery device adapted to dispense a particularly small dose or multiple small doses of a medicinal product. The drive mechanism should be easy and intuitive in handling and should provide an accurate and fine-tuned dose setting. It is further an object of the invention, to provide a drive mechanism and a drug delivery device comprising a simple internal structure, which is inexpensive to produce as well as fast and easy to assemble. Additionally, the drug delivery device and its dose dispensing mechanism should be mechanically stable and robust.